1. Field of the Invention
The present invention relates to an optical pickup apparatus used in optical data recording/reproducing, and particularly to an optical pickup apparatus having a relatively small size and high performance.
2. Discussion of the Background
Generally, in an optical pickup apparatus, a light beam emitted from a semiconductor laser diode of a light source is transmitted through a diffracting device, and the light beam is converted to a parallel light by a collimating lens and is irradiated onto a surface of an optical recording medium by an objective lens. A light beam reflected from the surface of the optical recording medium is returned via the same optical path as that of the irradiated light, is diffracted by the diffraction device, and is received on a photodetecting device so that signal light detection is executed.
During this signal light detection, unless intensity of the light beam from the semiconductor laser diode is constant, accurate signal light detection cannot be executed. Therefore, in conventional optical pickup apparatuses, the power of the emitted light beam has been monitored.
In one such optical pickup apparatus, in Japanese Patent No. 2543237 for example, an optical disk apparatus is proposed which includes a laser light emitting device serving as a light source, and a first optical device which focuses a light emitted from the laser light emitting device on to a data recording surface serving as an optical disk. A second optical device includes a first region for generating a first diffracting light of a transmitting type hologram, which generates a first diffracting light corresponding to a focusing error or a tracking error in an optical pickup with respect to the data recording surface of the optical disk on the basis of a light reflected from the data recording surface, and a second region for generating a second diffracting light of a reflection type hologram, the second region being coated on its surface by a film of a predetermined reflectance and which generates a second diffracting light corresponding to a monitor light by reflecting and diffracting a part of the light emitted from the laser light emitting device toward the data recording surface. A photodetector detects the first and the second diffracting lights generated by the second optical device, and a focusing controlling device focuses the light beam according to an output of the photodetector. A tracking controlling device tracks a predetermined position according to an output of the photodetector, and an output controlling device controls an output of the laser light emitting device to a predetermined value according to an output of the photodetector.
Namely, in the above described optical disk apparatus, the second optical device is provided with a first region for generating a first diffracting light which generates the first diffracting light, corresponding to a focusing error or a tracking error of an optical pickup with respect to a data recording surface of the optical disk on the basis of a light reflected from the data recording surface, and a second region for generating a second diffracting light, the second region being coated on its surface by a film of a predetermined reflectance and which generates a second diffracting light corresponding to the monitor light by reflecting and diffracting a part of a light emitted from a laser emitting device toward the data recording surface. Further, the first diffracting light and the second diffracting light which are generated in the second optical device are detected on a photodetector, and the output of the laser light emitting device is controlled by using the output of the photodetector of this optical disk apparatus. Thereby, the optical pickup apparatus is made small in size and at low cost.
Also, a technique to increase light utilization efficiency and to obtain a signal of high signal to noise ratio by using a polarization hologram has been proposed (see “HOE unit of high efficiency for DVD (2)” in Applied Physics Society in spring of 1998).
In this technique, signal light detection is executed by using a polarization hologram, and further a reflection hologram, which is not a polarization hologram but a general hologram and formed beside the polarization hologram, for detecting a monitor light The reflection hologram is formed on a board which is separate from the board of the polarization hologram for signal light detection, and the two boards are aligned in position, pasted, and integrated to each other.
However, in such conventional techniques, in order to further reduce size and lower cost, and also to detect a signal further accurately, further improvement has been necessary.
Namely, in the optical disk apparatus disclosed in Japanese Patent No. 2543227, the second optical device is provided with a first region for generating a first diffracting light of a transmitting type hologram which generates a first diffracting light corresponding to a focusing error and a tracking error in an optical pickup with respect to a data recording surface on the basis of the light reflected from an optical recording surface, and a second region, for generating a second diffracting light of a reflecting type hologram, which is coated on its surface by a film of a predetermined reflectance and which generates a second diffracting light corresponding to a monitor light by reflecting and diffracting a part of a light emitted from a laser light emitting device toward the data recording surface. If the optical pickup apparatus is configured such that a transmitting type hologram and a reflecting type hologram are used for increasing the light utilization efficiency and for obtaining a signal of high signal to noise ratio and both holograms exist together on one board for reducing the cost of the apparatus, optimum processing can be executed only for one of the holograms. Therefore, there are problems in that light utilization efficiency and accuracy of signal light detection are reduced.
Further, in the technique disclosed in the HOE unit of high efficiency for DVD (2), the signal light detection is executed by using a polarization hologram, and further a reflection hologram, which is not a polarization hologram but a general hologram and formed beside the polarization hologram, for detecting a monitor light. With this technique, there are problems in that diffraction efficiency of a monitor light is about 20% at most, and that flare light is large. In addition, because the reflection hologram is formed beside the polarization hologram, and alignment of the positions of the polarization hologram and the reflection hologram is required to be executed with high accuracy, the manufacturing efficiency is relatively low, the size of an apparatus is relatively large, and the cost of the apparatus is relatively high.